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Investments (Selected Information Regarding Marketable Equity and Debt Securities) (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Proceeds from sales of securities |  | $ 457 | $ 377 | $ 1,301 | $ 1,496
Realized gains | [1] | 24 | 25 | 96 | 142
Realized losses | [1] | 18 | 16 | 60 | 52
Virginia Electric and Power Company |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Proceeds from sales of securities |  | 237 | 156 | 651 | 654
Realized gains | [1] | 11 | 9 | 44 | 64
Realized losses | [1] | $ 5 | $ 6 | $ 17 | $ 24
[1] | Includes realized gains and losses recorded to the nuclear decommissioning trust regulatory liability.

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